The prior art relating to vehicular electrical systems is replete with dual voltage power supplies of various forms.
One system presently in production provides power at sixty volts to an electrically heated windshield. This is accomplished using a conventional alternator designed for a nominal twelve volt system. When windshield heating is required the alternator is disconnected from all of the nominally twelve volt systems and is connected with the windshield heating element. The alternator field is then energized to generate the required output voltage. For a period of time, all of the nominally twelve volt systems are energized from the vehicle battery while the alternator supplies power to the windshield. If the battery voltage drops below a predetermined level while the windshield is being heated a protective circuit disconnects the windshield from the alternator and restores alternator power to the other vehicle systems. The resistance of the windshield is monitored by a resistance bridge in which the windshield and a current sensing resistor connected in series comprise one side of the bridge.
This known system has the disadvantage of requiring a large mechanical relay which is a type of device that is well known to be less reliable than semiconductor switches. This known system also has the disadvantage of requiring the battery to supply the vehicle, which may require large amounts of power, at all times that the windshield is being heated. This requires a large and expensive battery which is still inadequate to meet some user demands. This known system also has the disadvantage of requiring a current sensing resistor capable of carrying large currents which is expensive and wasteful of energy.
U.S. Pat. No. 4,604,528 issued Aug. 5, 1986 to Peter Norton teaches a dual voltage system wherein the alternator operates at a higher voltage at all times and a DC to DC converter converts some of the higher voltage power to a lower voltage so all vehicle systems can be powered simultaneously.
My copending application Ser. No. 941,332 filed Dec. 15, 1986 for "Dual Or Single Voltage Vehicular Power Supply With Improved Switch" teaches a dual voltage regulator using P-channel mosfet transistors which can operate in both dual voltage and single voltage modes and which has a main gate drive circuit which drives the gate electrode of the main power switch through a capacitor. An augmenting circuit maintains the main power switch in its on or off state indefinitely which the main gate drive circuit could not do because the capacitor would eventually discharge. The disclosure of that patent application is incorporated herein by reference.
Circuits for driving mosfet power transistors are described in the literature. A review of such circuits is provided by chapter 6 of the second edition of POWER MOSFET TRANSISTOR DATA copyright 1986 and published by Motorola Inc. Known circuits suitable for driving an N-channel power mosfet transistor operating as a high side switch require substantial power at a voltage above the voltage of the high rail or cannot maintain the transistor in the on state indefinitely.
When a load device is disconnected from an alternator the output voltage of the alternator can rise rapidly, in the absence of protective equipment, to a voltage that may exceed one hundred volts. It is known to include transient absorbing semiconductors to limit the voltage rise. These devices must be much larger where the alternator operates at about six volts because much higher powers must be absorbed therefore resulting in higher cost.
Multilayer ceramic capacitors have recently become available in capacitances in the range of ten to one hundred microfarads which makes them suitable for storing energy in vehicular dual voltage electrical systems. These capacitors are relatively expensive thereby encouraging circuit designs that minimize the capacitance of the energy storage capacitor.
A general object of this invention is to provide a dual voltage electrical system for automotive vehicles which overcomes certain disadvantages of the prior art.